Sphenoid, positive and negative

Description

"It consists of four isoceles triangular faces which intersect all three of the crystallographic axes, the intercepts on the two horizontal axes being equal. The faces correspond in their position to the alternating faces of the tetragonal pyramid of the first order. There maybe different sphenoids, depending upon their varying intersections with the vertical axes. There may also be a positive and a negative sphenoid, the combination of the two being represented." — Ford, 1912